Liquid CO2 Pre-Fracture Permeability Enhancement Research in High Gas Content Working Face of Sizhujia Study Notes

Literature Overview and Core Focus

This study investigates the application of liquid carbon dioxide (CO₂) injection for pre-fracture permeability enhancement in high-gas-content coal mine working faces, specifically at the Sizhujia mining area. While this topic belongs to the domain of mining engineering and coalbed methane extraction rather than cladding or pressure vessel fabrication, the underlying principles of fracture mechanics, gas-liquid phase transitions, and pressure vessel design for high-pressure CO₂ storage and injection systems have significant relevance to pressure vessel engineers. The design and qualification of high-pressure vessels for liquid CO₂ storage and injection systems involves challenges directly related to bimetal pressure vessel fabrication, particularly regarding low-temperature service and hydrogen-induced cracking resistance.

Technical Approach and Process Parameters

Liquid CO₂ Injection System Design

The study describes a liquid CO₂ injection system that utilizes the phase transition from liquid to supercritical or gaseous CO₂ to generate fracturing pressure within coal seams. The key process parameters include:

Fracture Mechanism

The liquid CO₂ injection creates fractures through a combination of (1) high-pressure injection exceeding the coal seam minimum horizontal stress, (2) rapid phase transition from liquid to gas upon depressurization, which creates a volume expansion of approximately 500 times, and (3) thermal effects from the Joule-Thomson cooling during expansion, which creates thermal stresses in the coal matrix.

Pressure Vessel Considerations for CO₂ Injection Systems

Material Selection for High-Pressure CO₂ Service

The design of pressure vessels for liquid CO₂ storage and injection systems presents unique material challenges that are directly relevant to bimetal pressure vessel engineering:

Low-Temperature Service Requirements

Liquid CO₂ at its boiling point (-78.5 °C at atmospheric pressure) or under pressure (-40 to -20 °C) imposes severe low-temperature requirements on pressure vessel materials. The impact toughness requirements specified in GB/T 150 and ASME VIII Div.1 must be met at the minimum design temperature. For carbon steel vessels, the Charpy V-notch impact energy must exceed 47 J at the design temperature, which typically requires the use of low-temperature grades such as 16MnDR (Chinese standard) or A516 Gr.70 (ASME standard).

Hydrogen-Induced Cracking (HIC) and Sulfide Stress Cracking (SSC) Considerations

CO₂-containing environments can generate carbonic acid, creating a corrosive environment that may promote HIC and SSC in susceptible carbon steel materials. The NACE MR0175/ISO 15156 standard provides guidelines for material selection in such environments. Overlay protection with nickel-based alloys (Inconel 625, Hastelloy C276) or duplex stainless steels provides effective protection against CO₂ corrosion and HIC.

Connection to Bimetal Pressure Vessel Fabrication

The pressure vessels used in liquid CO₂ injection systems are excellent candidates for bimetal fabrication using weld overlay technology. The combination of a carbon steel base with a corrosion-resistant overlay provides the optimal balance of structural strength, low-temperature toughness, and corrosion resistance. Key fabrication considerations include:

1. Overlay material selection: Inconel 625 (UNS N06625) provides excellent resistance to CO₂ corrosion and HIC, while maintaining good low-temperature toughness.
2. Overlay thickness: Minimum 3 mm overlay is recommended for CO₂ service, with 6 mm preferred for severe conditions.
3. Heat treatment: Post-overlay stress relief at 850–900 °C for austenitic overlays to minimize residual stresses that could promote HIC.
4. Inspection requirements: 100% ultrasonic testing (UT) of overlay bonds and 100% magnetic particle testing (MT) of overlay surfaces per NB/T 47014 and JB/T 4730.

Key Questions and Reflections

A critical question for pressure vessel engineers is how to ensure the long-term integrity of bimetal pressure vessels operating in liquid CO₂ service, where the combination of low temperature, high pressure, and corrosive environment creates a challenging service condition. The answer lies in rigorous material selection, careful fabrication practices, and comprehensive inspection protocols.

Another important reflection is the role of fracture mechanics in predicting the service life of pressure vessels in CO₂ environments. The combination of thermal cycling (from -40 °C to ambient temperature during injection cycles) and corrosive exposure can lead to fatigue crack initiation and growth. Engineers must consider fatigue life assessment in addition to fracture toughness requirements when designing pressure vessels for cyclic CO₂ service.

Study Insights and Implications

The fundamental insight from this study, when viewed from a pressure vessel engineering perspective, is that the emerging applications of liquid CO₂ injection in mining and energy sectors create new demands for high-pressure, low-temperature bimetal pressure vessels. These vessels must meet stringent requirements for structural integrity, low-temperature toughness, and corrosion resistance simultaneously. The application of advanced weld overlay technologies — particularly wide-band ESW and SAW overlay with nickel-based alloys — provides an effective solution for fabricating such vessels. Engineers must develop comprehensive qualification programs that address the unique combination of mechanical, thermal, and corrosion challenges posed by liquid CO₂ service.